…ad records during load operation

When executing util.importTable() or util.loadDump(), if LOAD DATA
statement finished with warnings, Shell would print on screen/write to
the log file messages like:

  Warning: schema@table@123.tsv.zst error 1062: Duplicate entry
  '1234567' for key 'table.PRIMARY'

It was difficult to locate these messages in the log file, as they
lacked some common prefix that would allow to i.e. easily grep for them.

The message has been improved:

  Warning: An error has been reported while loading data into
  `schema`.`table` from 'schema@table@123.tsv.zst' file, error 1062:
  Duplicate entry '1234567' for key 'table.PRIMARY'

Change-Id: I0ffc43e9af45fb14e92fd3c9a5714c4622da3e51

Change-Id: Ic6c7ee1593327be45307442e36a5fdfb12aad910